1. Field of the Invention
The present invention relates to a switching power supply device wherein jitter is applied to the switching frequency, reducing the occurrence of noise.
2. Description of the Background Art
FIG. 10 shows an example of a typical configuration of a switching power supply device (AC/DC converter) that converts an alternating current voltage into a predetermined direct current voltage. The switching power supply device includes, in brief, a switching power supply device main body 1 formed of a switching element (for example, a MOSFET) 4 connected via a primary coil (inductor) 3a of a transformer 3 to a rectifier circuit 2 that rectifies an input alternating current voltage. The switching element 4, when turned on, performs a role of forming a current path between the switching element 4 and the rectifier circuit 2 passing through the primary coil (inductor) 3a. Also, an output capacitor 6 is connected via a diode 5 to a secondary coil 3b of the transformer 3. The diode 5 performs a role of rectifying a voltage generated by the secondary coil 3b of the transformer 3 when the switching element 4 is turned off, thus charging the output capacitor 6, and generating a predetermined output direct current voltage Vout via the output capacitor 6.
Reference sign 10 in the drawing is a control circuit (control IC) that drives the switching element 4 on and off, thus controlling current flowing through the primary coil 3a of the transformer 3. Also, reference sign 8 is an output voltage detector circuit that detects the difference between the output direct current voltage Vout detected via resistors Ra and Rb and an output set voltage. The voltage difference detected by the output voltage detector circuit 8 is provided via a photocoupler 9 to the control circuit 10 as a feedback voltage FB. Further, the control circuit 10, basically, controls the switching frequency of the switching element 4 in accordance with the feedback voltage FB, thereby evening the output direct current voltage Vout.
FIG. 11 is a main portion schematic configuration diagram showing an example of the control circuit 10. The control circuit 10, in brief, includes an oscillator circuit (OSC) 11, which oscillates at a frequency in accordance with the feedback voltage FB, and an output buffer 12 that drives the switching element 4 on and off in accordance with the output of the oscillator circuit 11. Also, the control circuit 10 includes a start-up circuit 13 that detects the input alternating current voltage, which is applied to a terminal VH, starts up an internal power supply generator circuit (not shown), and starts up the output buffer 12. The internal power supply generator circuit and output buffer 12, after being started up by the start-up circuit 13, operate by a voltage generated by an auxiliary coil 3c of the transformer 3 being input from a terminal VCC.
Furthermore, the control circuit 10 includes an overcurrent detector circuit 14 that detects an overcurrent by current flowing through the switching element 4 being input via a terminal CS, and includes an overheat detector circuit 15 that detects an overheat of the control circuit 10 via a terminal LAT. The overcurrent detector circuit 14 and the overheat detector circuit 15 cause the drive of the switching element 4 to stop by prohibiting the operation of the output buffer 12. Because of this, the switching power supply device main body 1 is protected from overcurrent and overheat trouble. As these protective functions are not directly related to the invention, a detailed description thereof will be omitted.
Continuing, the oscillator circuit 11 in the control circuit 10 includes a buffer amplifier (AMP1) 11a that detects the feedback voltage FB, and an amplifier (AMP2) 11b that controls current flowing through a transistor (n-channel MOSFET) q1 in accordance with the output of the buffer amplifier 11a. The transistor q1 includes as a load a current minor circuit formed of transistors (p-channel MOSFETs) q2 and q3. An output current of the current minor circuit is provided to a transistor (n-channel MOSFET) q4, which acts as a load of the current minor circuit, and used to control current flowing through a transistor (n-channel MOSFET) q10. Furthermore, the output current of the current minor circuit is used to control current flowing through a transistor (p-channel MOSFET) q7 via a transistor (n-channel MOSFET) q5 and a transistor (p-channel MOSFET) q6.
The transistors q7 and q10 are connected in series via transistors (p-channel and n-channel MOSFETs) q8 and q9, which are controlled so as to be turned on and off in a complementary way. Further, a capacitor C is connected to a series connection point of the transistors q8 and q9. The transistor q8 performs a role of charging the capacitor C using current flowing through the transistor q7 when carrying out an on-state operation. Also, the transistor q9 performs a role of discharging the capacitor C using current flowing through the transistor q10 when carrying out an on-state operation.
Further, a hysteresis comparator 11c compares the charging and discharging voltages of the comparator C and a predetermined reference voltage Vref, and generates a pulse signal that forms a reference for driving the switching element 4 on and off. The output (pulse signal) of the hysteresis comparator 11c is provided to the output buffer 12, and the switching element 4 is driven on and off by the output buffer 12. Also, at the same time, the output of the hysteresis comparator 11c is used as a control signal that drives the transistors q8 and q9 on and off in a complementary way, and as a clock signal that regulates the action of a jitter control circuit 20.
Herein, the jitter control circuit 20 includes a plurality (four) of transistors (p-channel MOSFETs) q11 to q14 that form a parallel current mirror circuit with the transistor q7, and transistors (p-channel MOSFETs) q21 to q24 connected in series to the transistors q11 to q14 respectively, as shown in, for example, FIG. 12. The transistors q21 to q24 are controlled so as to be turned on or off by receiving outputs Q0 to Q3 of an up/down counter 21, and perform a role of selectively drawing current flowing through the transistors q11 to q14, and applying the current to the transistor q5.
The currents flowing through the transistors q11 to q14 are set as, for example, I1, I2 (=2·I1), I3 (=2·I2=4·I1), and I4 (=2·I3=4·I2=8·I1) respectively. These current ratios are set by changing the area ratios of the transistors q11 to q14, each of which forms a current mirror circuit with the transistor q7.
Also, the up/down counter 21 receives the output of a divider 22 that divides the output (pulse signal) of the hysteresis comparator 11c, and carries out a counting operation. Further, the up/down counter 21 alternately repeats operations of counting up (increment of +1) and counting down (decrement of −1) a count value thereof. As a result of this, for example, the 4-bit up/down counter 21 causes the outputs Q0 to Q3 thereof to vary sequentially and reversibly within a range of, for example, [0000] to [1111]. The transistors q21 to q24 are selectively controlled so as to be turned on or off by the outputs Q0 to Q3 of the up/down counter 21. Further, the currents flowing through the transistors q11 to q14 are selectively output by a selective on-state operation of the transistors q21 to q24.
As a result of this, an output current b of the jitter control circuit 20 varies in step form (triangular wave form), and the output current b is applied to the transistor q4. Further, a step-form (triangular wave-form) variation is applied to the current that charges the capacitor C, and a cyclical variation is applied to the time for charging the capacitor C as far as the reference voltage Vref. As a result of this, cyclical fluctuation of a constant width is applied to the frequency of the pulse signal output via the hysteresis comparator 11c. Control of this kind of oscillation frequency is jitter control of the switching frequency that drives the switching element 4. Further, the frequency of high frequency noise that occurs in accompaniment to a switching of the switching element 4 is diffused by the jitter control, because of which the high frequency noise is reduced (for example, refer to U.S. Pat. No. 6,249,876).